Although didn't River give the Doctor some of her own regeneration energy or something? If so... wouldn't that kind of set his own clock back as it were?

That's certainly one possible "out" for the writers when they hit that 13th Regeneration and want to continue and also a way for The Doctor to legitimately be surprised when Matt Smith's second sucessor lays down to die a final death and instead pops back up as Doctor #14 "Well that wasn't supposed to happen....Ooh, and I'm finally Ginger "